1. Field of the Invention
The present invention relates generally to an electron gun for a cathode ray tube to be employed such as in television sets and display monitors. More specifically, the invention relates to an electron gun which is assembled in a novel process to provide increased uniformity of cut-off voltage.
2. Description of the Background Art
As is well known, electron guns comprise a plurality of grids arranged in alignment along their axes at predetermined intervals. The cut-off voltage E.sub.KCO of such electron guns can be determined according to the following formula: ##EQU1## where d.sub.01 is the distance between a cathode K and a first grid electrode which is the closest grid electrode to the cathode; and
d.sub.12 is the distance between the first grid electrode and the second grid electrode which is located adjacent the first grid electrode.
As will be appreciated, when assembling electron guns the aforementioned d.sub.01 and d.sub.12 distances must be maintained accurately in order to obtain high uniformity of the cut-off voltage E.sub.KCO. It is especially essential to precisely maintain the predetermined distance between the cathode and the first grid electrode.
In conventional assembly processes for assembling electron guns, mounting pins are provided for respective grid electrodes at radially symmetrical positions or integrally formed projections are provided projecting from the outer periphery of the grid electrode at radially symmetrical orientations. The mounting pins and projections are buried in a pair of glass beads which are located at respective axial positions while predetermined distances are maintained between the grid electrodes. The glass beads thus maintain the distance between the grid electrodes as they are assembled. After forming the aforementioned assembly of the grid electrodes and the glass beads, a cathode base with cathode or cathodes is mounted on the first grid electrode with a predetermined spacing to the latter maintained by means of an appropriate spacer. Then, the distance between the plane of the first grid electrode, in which an electron beam path in the form of a through opening is defined, and the mating end of the cathode is adjusted while monitoring the d.sub.01 distance by means of an airmicrometer.
In the foregoing conventional process, spacers defining the predetermined distances between grid electrodes are inserted between respective grid electrodes for defining the distance between the respectively adjacent grid electrodes upon forming the assembly of the glass beads and the grid electrodes. The grid electrodes and the spacers are held tightly in their assembled positions by means or a jig. At this position, the glass beads are opposed to the mounting pins or projections at radially symmetrical positions. Then, the glass beads are heated and depressed so as to bury the mounting pins or the projections. In this way the grid electrodes are mechanically connected via the glass beads. In this process, the distance between the first and second grid electrodes are adjusted to the predetermined distance in order to obtain the predetermined cut-off voltage. However, in practice, since the first and second electrodes are depressed during the beading process the distance therebetween is altered during the process. Therefore, in production, uniformity of the cut-off voltage of the electron guns has been difficult to obtain.
In another process, the dimension (d.sub.01 +d.sub.12) between the cathode surface to the second grid electrode is measured by means of an airmicrometer inserted from the side of the second grid electrode. According to the result of this measurement, the position of the cathode base is adjusted. In this process, though the dimension d.sub.01 +d.sub.12 can be adjusted in a rather precise manner, it is not possible to adjust respectively the dimensions, d.sub.01 +d.sub.12, separately. As will be seen from the foregoing formula, the dimension between the cathode surface and the first grid electrode will have greater influence than that of between the first grid electrode and the second electrode. Therefore, even with this process, uniformity of the cut-off voltage cannot be obtained.
In other conventional assembly processes for assembling an electron gun, a pre-assembly of the first grid electrode and the cathode is prepared before the beading operation. During preparation of the grid/cathode pre-assembly, the distance between the cathode surface and the first grid can be precisely adjusted to the predetermined distance d.sub.01 by utilizing the airmicrometer. The prepared grid/cathode pre-assembly is subjected to the beading process along with the other grid electrodes in substantially the same manner as that described above. In this process, the dimension d.sub.01 can be obtained precisely in the pre-assembly process. However, the problem of deformation of the first and second grid electrodes as discussed with respect to the first process remains unsolved.
On the other hand, in recent years, the demand for fine pitch type cathode ray tubes has increased. This requires high precision adjustment of the cut-off voltage and consequently requires high production accuracy of the electron gun.
The following applications relating to electron guns are assigned to the Assignee of the present application:
1. Ser. No. 802,476, Electron Gun For a Color Display Apparatus, filed Nov. 27, 1985, Inventors: Iguchi et al;
2. Ser. No. 047,578, Electron Gun, filed May 8, 1987, Inventor: Watanabe;
3. Ser. No. 867,535, Electron Gun, filed May 28, 1986, Inventor: Yasuda; and
4. Ser. No. 826,836, Electron Gun, filed Feb. 6, 1986, Inventor: Watanabe.